1. Field of the Invention
The present invention generally relates to cutting elements for downhole cutting tools. More specifically, the present invention relates to composite materials for cutting elements of downhole cutting tools, such as rock bits, which enhance the useful life of the cutting tools, and cutting tools incorporating the same.
2. Background Art
Conventional drilling systems used in the oil and gas and mining industries to drill wellbores through earth formations include a drilling rig used to turn a drill string which extends downward into a well bore. A drill bit is typically connected to the distal end of the drill string and is designed to break up earth formation in its path when rotated under an applied load. Typically, drilling fluid or air is pumped through the drill pipe and drill bit to move cuttings away from the bit during drilling and up an annulus formed between the drill string and the borehole wall.
Earth boring drill bits generally are made within one of two broad categories of bit structures. Drill bits in the first category are generally known as “fixed cutter” or “drag” bits, and usually include a bit body formed from steel or another high strength material and a plurality of cutting elements disposed at selected positions about the bit body. The cutting elements may be formed from any one or combination of hard or superhard materials, including, for example, tungsten carbide, natural or synthetic diamond, and boron nitride.
Drill bits of the second category are typically referred to as “roller cone” bits, and include a bit body formed from steel or another high strength material and having one or more roller cones rotatably mounted on the bit body. The roller cones are also typically formed from steel or other high strength material and include a plurality of cutting elements disposed at selected positions about the cones. The cutting elements may be formed from the same base material as the cone. These bits are typically referred to as “milled tooth” bits. Other roller cone bits include cutting elements, referred to as “inserts,” which are press (interference) fit into holes formed and/or machined into the roller cones. The inserts may be formed from, for example, tungsten carbide, natural or synthetic diamond, boron nitride, or any one or combination of hard or superhard materials.
Due to its toughness and high wear resistance, cemented tungsten carbide is widely used to form cutting elements in rock-drilling and earth boring applications. “Cemented tungsten carbide” generally refers to a tungsten carbide composite which comprises tungsten carbide (“WC”) grains bonded together by a binder phase. In most applications, the binder phase comprises cobalt (Co), nickel (Ni), and/or iron (Fe). Tungsten carbide grains dispersed in a cobalt binder matrix is the most common form of cemented tungsten carbide currently used for cutting elements in drilling applications, and is typically classified by grades based on the grain size of the tungsten carbide particles used and the cobalt content. However, in some cases, cemented tungsten carbide may be classified by grades based on the cobalt content and a material property such as hardness or wear resistance.
In general, cemented tungsten carbide grades are primarily made in consideration of two factors that influence the lifetime of a tungsten carbide insert: wear resistance and toughness. As a result, conventional tungsten carbide grades used for cutting elements of downhole drilling tools have cobalt contents of 6% to 16% by weight and tungsten carbide “relative” particle size numbers of 3 to 6 (which equates to an average tungsten carbide grain sizes of less than 3.0 microns (μm), as measured by the ASTM E-112 method). These conventional grades typically have a Rockwell A hardness of between 85 and 91 Ra, a fracture toughness below 17 ksi(in)0.5 (as measured by the ASTM B-771 method) and a wear number between 1.8 to 5.0 (as measured by the ASTM B-611 method). In particular, these grades are widely used for inserts forming interior rows on roller cone bits.
Gage row inserts are often selected to have a higher wear number than interior row inserts because it is generally believed that gage inserts need higher wear resistance due to the large amount of borehole wall contact they encounter during drilling. As a result, the toughness of gage inserts is typically sacrificed to gain wear resistance. However, this practice improperly assumes that the rock to be drilled by the gage inserts generally has the same properties in every application. In many applications, this is not the case and this practice has led to the breakage of gage inserts with the interior rows still intact.
For example, when drilling softer formations, such as carbonates, the wear resistance of inserts is not the major concern because these formations are not very abrasive. Rather, resistance to thermal fatigue and heat checking has been found to be the primary concerns that result in premature cracking and breakage of inserts. This occurs because the tungsten carbide inserts of a rock bit are subjected to high wear loads from contact with a borehole wall, as well as high stresses due to bending and impact loads from contact with the borehole bottom. These high wear loads can lead to thermal fatigue of the inserts which, in turn, leads to the initiation of surface cracks (referred to as heat checking) on inserts. These surface cracks are then propagated by a mechanical fatigue mechanism caused by the cyclical bending stresses and/or impact loads applied to the inserts during drilling. The result is chipping, breakage, and/or failure of inserts which shortens the useful life of the drill bit.
In particular for roller cone drill bits, inserts that cut the corner of a borehole bottom are often subjected to the greatest amounts of thermal fatigue due to heat generation on the inserts from a heavy frictional loading component produced as the inserts engage the borehole wall and slide into their bottom-most crushing position. As the cone rotates, the inserts retract from the borehole wall and are quickly cooled by circulating drilling fluid. This repetitive heating and cooling cycle can lead to the initiation of surface cracks on the inserts (i.e., heat checking). These cracks are then propagated through the body of the insert as the insert repeatedly impacts the borehole wall and high stresses develop.
The time required to progress from heat checking to chipping, and eventually, to breakage of inserts depends upon several factors including the formation type, rotation speed of the bit, and applied weight on bit. In many applications, especially those involving higher rotational speeds and/or higher weights on bit, thermal fatigue and heat checking of inserts are issues that have not been adequately addressed. Consequently, inserts made of standard tungsten carbide grades have been found to frequently fail in these applications.
To help reduce insert failures caused by thermal fatigue and heat checking, coarser grain carbide grades have been proposed for cutting elements of drill bits. Examples of grades proposed are further described in U.S. Pat. Nos. 6,197,084, 6,655,478, 7,017,677, 7,036,614, 7,128,773, and U.S. Publication No. 2004/0140133 A1, which are all assigned to the assignee of the present invention and incorporated herein by reference. These grades comprise coarse carbide grains having average grain sizes greater than 3.0 μm and binder contents of 6 to 16% by weight. Inserts formed from these composite materials have been found to exhibit higher fracture toughness and adequate wear resistance for many drilling applications. These inserts have been shown to result in improved performance and/or longevity when compared to inserts formed of conventional carbide grades. In particular, coarser grain composites have been found to be particularly useful in reducing gage carbide failures due to heat checking.
While improvements in bit life have been seen, premature breakage and failure of inserts has still been found to occur in some applications. Accordingly, a desire exists for improved composite materials that provide enhanced thermal fatigue and shock resistance with adequate wear resistance for these drilling applications to help further improve drill bit life.